Prevent SubViewports Update Mode from resetting on visibility changes

[Deltt]

Removes potentially undesired resets of render_target_update_mode on visibility changes.

see #23729 and #71227

First commit btw so I assume I didn't do something correct or informaly, please let me know how...

I think this line has either been an oversight or is simply a leftover from before other render_target_update_modes were implemented or planned.